Revert "(namespace) Revert "UBUNTU: SAUCE: fs: Refuse uid/gid changes which don't...

Revert "(namespace) Revert "UBUNTU: SAUCE: fs: Refuse uid/gid changes which don't map into s_user_ns""

BugLink: https://bugs.launchpad.net/bugs/1644165
This reverts commit 412b16efeff033fe3f14a03f1511244785f59a60.

The kernel fix for bug #1634964 breaks LXD userspace, in particular the
following commits:

ac7f3f73cb39 (namespace) vfs: Don't modify inodes with a uid or gid unknown to the vfs
ca52383ad6a6 (namespace) vfs: Don't create inodes with a uid or gid unknown to the vfs

LXD 2.0.6 will include changes to support these kernel changes, but it isn't
available yet on xenial, so for now we just revert these commits.

Signed-off-by: Luis Henriques <luis.henriques@canonical.com>